It is well known to multiplex a plurality of signals for transmission via a single transmission medium. In particular for digital systems, this is done by assigning a predetermined proportion of the total bitrate available in the transmission medium to each of the signals that it is desired to transmit. The proportion of the available bitrate assigned to each signal is fixed in each system.
These types of systems are often referred to as Variable Quality Fixed Bitrate systems; FIG. 2 presents an illustration of such a system. The FIG. 2 system includes a number of encoders 20, 20.sub.1, 20.sub.2 . . . 20.sub.n connected to a Multiplexer (MUX) 22. A video signal is transmitted through each of the encoders such that a plurality of fixed bitrate signals 24, 24.sub.1, 24.sub.2 . . . 24.sub.n are transmitted to the MUX 22. Each video signal, by way of typical example, constitutes a different television channel. A Multiplex Control Computer (MCC) 26 controls the output from the MUX so that a satellite signal 28 is generated which includes all of the television channels. The satellite signal is then transmitted, via a satellite (not shown), to a receiver station (not shown) which receives the satellite signal and decodes it back into the plural different television channels. The full details of the decoding operations do not form a part of the present invention and are accordingly not described in detail herein.
One problem with such systems is associated with the fact that, in practice, some video pictures require a much higher bitrate than others to achieve acceptable picture quality. In the above-mentioned arrangement, for example, it may be that the signals from some picture sources or television channels cannot be properly encoded using the available bitrate, while others do not require all of the bitrate capacity that is available to them.
In the conventional Fixed Bitrate, Variable Quality system, encoders are allocated a bitrate by the MCC as described above. This bitrate is then fixed for the duration of the MCC schedule. Thus, an encoder in the fixed bitrate system cannot respond to changes in criticality of the source video, as a result of which the picture quality of the decoded output of a fixed bitrate encoder varies with the criticality of the picture material. As the picture material becomes more demanding (i.e. contains more motion and complexity), the picture quality will decrease and the decoded output will contain more encoding artifacts. This is clearly undesirable.
One object of the present invention is to therefore provide a system or arrangement which overcomes at least some of the limitations of known systems.